Now, spinning CDs to clean sewage water

Wonder what to do with your obsolete audio CDs?
Researchers have come up with a practical application: they can be used to break down sewage.
"Optical disks are cheap, readily available, and very commonly used," said Din Ping Tsai, a physicist at National Taiwan University.
Close to 20 billion disks are already manufactured annually, the researchers noted, so using old disks for water treatment might even be a way to cut down on waste.
Tsai and his colleagues used the large surface area of optical disks as a platform to grow tiny, upright zinc oxide nanorods about a thousandth the width of a human hair.

Zinc oxide is an inexpensive semiconductor that can function as a photocatalyst, breaking apart organic molecules like the pollutants in sewage when illuminated with UV light.

While other researchers have experimented with using zinc oxide to degrade organic pollutants, Tsai's team is the first to grow the photocatalyst on an optical disk.
Because the disks are durable and able to spin quickly, contaminated water that drips onto the device spreads out in a thin film that light can easily pass through, speeding up the degradation process.
The team's complete wastewater treatment device is approximately one cubic foot in volume. In addition to the zinc oxide-coated optical disk, the device consists of a UV light source and a system that recirculates the water to further break down the pollutants.

The research team tested the reactor with a solution of methyl orange dye, a model organic compound often used to evaluate the speed of photocatalytic reactions.
After treating a half-litre solution of dye for 60 minutes, they found that over 95 per cent of the contaminants had been broken down. The device can treat 150 ml of waste water per minute, the researchers said.
The spinning disk reactor is small, consumes little power, and processes contaminated water more efficiently than other photocatalytic wastewater treatment methods, Tsai said.
The device could be used on a small scale to clean water contaminated with domestic sewage, urban run-off, industrial effluents, and farm waste.

Going forward, the team is also working on ways to increase the efficiency of the reactor, and Tsai estimates that the system could soon be improved to work even faster, perhaps by creating layers of stacked disks.
The team will present its new wastewater treatment device at the Optical Society's (OSA) Annual Meeting in Orlando, Florida.